Producing paper or paperboard on the industrial scale involves a complicated process whereby an aqueous papermaking slurry that comprises lignocellulosic-derived fibers (including virgin and/or recycled pulp fibers) is mixed with various process additives such as acids, bases, alums, sodium aluminate, sizing agents, dry strength additives, wet strength additives, filler/pigment materials (e.g., kaolin clay, titanium dioxide, calcium carbonate, etc.), retention aids, fiber defloculants, defoamers, drainage aids, optical brighteners, dyes, opacifiers, deposit control agents, antimicrobial agents, other specialty chemicals, etc.). The thus-treated pulp slurry is introduced to a process where the slurry is dewatered to form an initial wet paper web, which is generally pressed to further remove water and consolidate the wet paper web. This pressed wet paper web is dried and further processed to produce a sheet of paper or paperboard.
The optical properties of many paper and paperboard products, such as opacity and brightness, are one of the key criteria for judging its qualities to the papermaker, converting operations and ultimately to the end user such as the printers. These optical properties have to be balanced with other desired attributes in the sheet, primarily the sheet's basis weight, bulking value and its strength properties when subjected to various stresses (e.g., tensile, burst and tear strength). The balance of these optical and physical sheet properties is often governed by the paper or paperboard grade and the end use of the product.
Another important factor in the production of paper is the overall cost to produce a particular grade of paper or paperboard. One method for lowering the cost of making paper, particularly for grades having critical opacity and brightness requirements is to substitute part of the fiber furnish with inorganic filler/pigment materials. One exemplary category of paper products wherein opacity and brightness properties are critical to their functionality are communication papers (e.g., fine papers, newsprint, magazine, lightweight coated, etc.). These inorganic filler or pigment materials may include kaolin clay, calcined clay, ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), talc, alumina trihydrate, amorphous silica & metal silicates and titanium dioxide, just to name a few. The addition of these filler/pigment additives can:
1) improve the formation and overall sheet structure by assisting in filling the void areas;
2) increase the opacity of the resulting sheet by increasing light scattering;
3) improve the physical characteristics of the sheet that assist with printing process—preventing the show through of print on one surface from the opposite side (as a consequence of the increased opacity they provide);
4) improve the brightness and whiteness properties of the sheet;
5) lower the overall cost of the sheet with a cheaper material than lignocellulosic fibers. However, in doing so they frequently increase the basis weight of the paper product; and
6) significantly bulk the sheet in the case of using highly structured mineral fillers such as calcined clays, structured silicas and the like.
The type of mineral filler or pigment that is used is often determined by the grade of paper that is being made, i.e., the degree of opacity and brightness, the cost of use, the resultant basis weight and strength properties required in the final paper product. High refractive index pigments, like titanium dioxide, are often used in printing and writing grades where high brightness and high opacity are needed. However, titanium dioxide is a very expensive inorganic pigment material and is often unsuitable for lower cost paper grades. Kaolin clay, calcined clay, PCC and GCC are lower cost alternatives to titanium dioxide, but all provide lower opacification power and brightness due to their lower refractive index. For the production of acid newsprint grades, calcined kaolin clay is often used due to its low cost to provide paper opacity and its improved compatibility versus PCC or GCC with acid wet-end papermaking systems. Typical addition levels that may be used of inorganic fillers or pigments, depending on the required opacification and paper grade being made can range from about 2 to 20% by weight.
For some paper grades, such as lightweight-coated (LWC) or directory grades, it is desired to have a sheet that has high opacification with a low basis weight (or grammage). Inorganic filler/pigments can be used in these applications; however, the drawback of inorganic filler/pigments is they provide opacification while disproportionately increasing the basis weight of the sheet because of their higher density values relative to cellulose fiber. Hence, it may not be possible to obtain the needed sheet opacity at the desired basis weight when using only inorganic mineral filler/pigments as the opacification additive.
Another drawback with inorganic filler/pigments is the amount that can be used. The addition of inorganic filler/pigments to the sheet, particularly at higher loadings, can cause a significant reduction of the sheet's strength properties. This is partially related to the interferences that the inorganic filler/pigments create with the fiber-to-fiber bonding, which is important in the development of paper strength. Yet, another drawback of inorganic filler/pigments is the abrasiveness nature of the filler/pigments. Inorganic filler/pigments have different degrees of abrasiveness (related to their crystal structure and hardness) and this abrasiveness can cause excess wear on the papermaking equipment, e.g., moving papermachine wires, pumping equipment, cutters, trimmer knives in the converting area, and the like.
The use of inorganic filler/pigments in papermaking usually requires these materials be made into an aqueous slurry dispersion, in which the filler/pigment slurry is applied and mixed with the aqueous fiber containing papermaking slurry prior to the papermachine. This also generally requires the inorganic filler/pigments to be made down into a workable slurry that can be stored, easily pumped and metered into the wet-end of the papermachine. Because the suspended, solid particles in these slurries have a tendency to settle, the filler/pigment slurries generally require constant agitation in their make-down tanks.
Another problem often associated with using inorganic filler/pigments in papermaking systems is their propensity to foul the papermachine wire and press felts. Fouling decreases the effectiveness of the papermachine to dewater the pulp slurry, thus requiring down time to clean and/or replace these papermachine equipment, and a resultant increase in the cost of producing the paper product.
As a result of the various problems identified above with using the inorganic filler/pigment based opacification aids, the papermaking industry is in need of new methods to improve and/or increase the opacity of various paper and paperboard grades whole optical properties are critical to their end-use functionality. Depending on the grade of paper to be made, the need for alternative opacification methods outside the use of mineral filler/pigments is being driven by the need to:                1) provide cost effective opacification to paper products without the aforementioned slurry handling, papermachine fouling and particle retention issues of inorganic fillers or pigments;        2) to provide equivalent opacity to current mineral or pigment filled sheets at a lower basis weight;        3) to improve the strength properties of high ash content grades of paper by replacing a portion of the mineral filler/pigment with an equivalent opacifying portion of quaternized alkanolamine fatty acid ester compound; and        4) to enable the production of higher opacity super calendered (SC) fine papers wherein a significant fraction of the opacifying benefits imparted by the opacification additive are maintained after calendering.        
Part of the new opacification technology need is being driven, in the case of newsprint, by new multicolor printing technologies that require the newsprint to have better opacity and ink holdout. To date, the traditional approach to increasing newsprint opacity has been with the use of inorganic fillers (e.g., calcined kaolin clay, precipitated calcium carbonate (PCC), etc.). Another approach has been the use of organic dyes, with or without these other traditional aids. The brightness, whiteness, coloring and opacity of paper can be impacted through the addition organic dyes. Certain organic dyes, such as black and blue dyes, can be used to increase sheet opacity; however, the amount of dye that is used must be balanced against decreasing the brightness of the sheet or tinting of the sheet to an off-white color, which may be undesirable. These dyes, depending on their type, are very sensitive to the wet-end chemistry of the papermachine, and can be sensitive to variations in the papermachine fiber furnish. The effectiveness of these dyes can also be negatively influenced if microbiological agents are used in the papermaking slurry, particularly oxidizers like chlorine, chlorine dioxide, peracetic acid, etc. Other disadvantages of dyes, compared to most inorganic filler/pigments, is their relative high cost and their impact on wastewater effluent streams from papermills, which may require some additional treatment to properly dispose of them.
Various other efforts besides the addition of dyes or inorganic mineral filler/pigments to the wet-end, have been made in the prior art to better control the optical properties of paper during papermaking. One approach is disclosed in U.S. Pat. Nos. 5,292,363, 5,296,024, 5,393,334, and 5,417,753 to Hutcheson, wherein organic based opacification aids as fatty amides of alkanoldiamines are used in papermaking. These opacification aids are produced via the reaction of various fatty acids and various alkanoldiamines. More particularly, the products result from the reaction of stearic acid with aminoethylethanolamine (AEEA) to form mono- and distearamides of AEEA. These products are generally water insoluble and solid waxy materials with high melting points (>75° C.) in their 100% active form. For the fattyamides of alkanoldiamines to be used as a paper opacification aid, these products are made into aqueous emulsions to improve their dispersion characteristics in papermaking slurries. These emulsions contain several additional substances and usually require high shear mixing. These additional substances can include surfactant(s) and viscosity controlling agent(s). To obtain stable emulsions for these agents, it is typical to employ low concentrations (generally 9-13%) of the fattyamides of alkanoldiamines. While primarily an opacification aid, Hutcheson also teaches that the aids can improve paper brightness and paper size.
Improvements to the organic opacification aids taught by Hutcheson are found in U.S. Pat. Nos. 5,472,486, 5,478,387, 5,488,139, 5,494,555, and 5,498,315 to Drager and North. Therein, the opacification formulation of Hutchenson is improved by the addition of certain additives to the fattyamides of alkanoldiamines to increase both paper opacity and paper strength (e.g., “glyoxyl compounds/block resins). Drager and North also expand the types of fatty acids that can be used for making the fattyamides of alkanoldiamines, e.g., dimerized and trimerized tall oil fatty acids.
The prior art has also proposed quaternized versions of fattyamides of alkanoldiamines, see U.S. Pat. No. 5,667,638 to Dragner and North. In this patent, a quaternized version of a fattyamide of alkanoldiamine such as alkyl bis (alkyl amido alkyl)-2-hydroxy alkyl ammonium alkali salt can be added to papermaking slurries to improve paper opacification. These organic compounds are usually soft paste materials at room temperature as compared to the waxy fattyamides of alkanoldiamines of the prior art discussed above.
Dragner and North also teach in U.S. Pat. No. 6,419,791 mixtures of natural fatty oils with various amine-esters as opacification aids in papermaking. The resulting compounds are not easily dispersed in water and must be formulated into emulsions with the concerns and characteristics discussed above regarding the Hutchenson patents. These species generally are less effective than the fattyamides of alkanoldiamines taught by Hutchenson.
While a number of organic based opacification aids for opacity relevant paper grades have been proposed by the prior art, a need still exists for improved aids, particularly in light of the problems noted above regarding the use of inorganic filler/pigments, dyes, a variety of quaternized fattyamides and the like. Commercial feedback on some of the quaternized fattyamides currently being used in paper mills as opacification aids has indicated that their low solids content emulsion forms, the high application doses needed for yielding good opacity, the resultant loses in sheet strength properties and accompanying papermachine deposit issues are significant end-user issues that need improvement.
The present invention responds to this need via the discovery that quaternized alkanolamine fatty acid esters can be employed in papermaking operations to provide improved optical performance as compared to the prior art organic opacification aids currently being used, e.g., fatty amides of alkanoldiamines, quaternized versions of these fatty amides, and mixtures of fatty oils and various amine-esters. Use of the quaternized alkanolamine fatty acid esters, hereinafter more simply referred to as diester quats, also provides control over other aspects of the papermaking operation, e.g., decreasing inorganic filler/pigment amounts for the purposes of improving strength properties and/or decreasing paper grammage without a loss in opacity.
Quaternized alkanolamine fatty acid ester compounds are known and their use in papermaking methods has been proposed in U.S. Pat. Nos. 5,217,576, 5,223,096, 5,240,562, 5,264,082, 5,415,737, and 5,427,696. Each of these patents centers around modifying paper properties in tissue and towel paper grades. This prior art teaches the use of various quaternary ammonium chemical softening compounds, which includes quaternized alkanolamine fatty acid esters. While this art teaches the use of these compounds as softening aids which impart a soft feel and more adsorbent paper in the stated paper areas, there is absolutely no recognition of the use of the quaternized alkanolamine fatty acid esters as a wet-end papermaking additive for improving opacity. In fact, opacity is not even an issue with these grades, since tissue and towel paper grades are not commonly used or designed for use in end-use applications where show-through, printing and writing performance are, for example, important.